This invention relates to various aspects of a tool useful for performing operations involving subsea christmas trees used by the oil and gas industry, and to various methods relating thereto.
Subsea christmas trees typically employ a specialized hydraulic clamping connector (typically called a tree connector) which latches and seals to a vertically oriented subsea wellhead housing which has been pre-installed on the sea bed. Functionally, the tree connector contains well pressure and serves to transmit external loading forces which are imposed on the subsea tree and ultimately the wellhead housing. These external loading forces are typically generated by a workover completion riser (tubulars) system which extends upwards from the subsea Christmas tree to an overhead floating workover completion vessel, typically a semi-submersible drilling rig. Other external forces may be imposed on the subsea tree due to items such as fishing trawls or flowline connection systems.
The connector typically seals to the wellhead housing by means of a metallic gasket which is energized between two mating conically shaped seal surfaces as the connector is latched to the wellhead housing. The connector latching system typically generates a vertical clamping force to the wellhead, typically called pre-load. The pre-load is generally a result of vertically applied force which is magnified by a mechanical advantage in a wedge-type clamping system. This pre-load force can vary in magnitude between designs, but ideally should be equal to or greater than the total combined equivalent tension on the connector. This is the summation of all axial forces generated by gasket compression, pressure end load, external tension, and external bending. The "ideal preload" requirement serves to prevent movement of the connector wellhead seal under load and ensure there is no metallic wellhead gasket damage on seal surfaces due to repetitive movements because of alternating stress cycles due to cyclical loading. Generally, the ideal preload requirement may not be required and may be of a significantly lesser value when cyclical loading is small. This has a direct bearing on the size and cost of a connector.
The subsea christmas tree connectors typically include a wellhead latching mechanism which is actuated by some form of integral hydraulic piston system. These systems vary from a number of discrete hydraulic cylinders to a single large diameter annular piston arrangement. The discrete piston system typically offers low pre-load because of limited piston area, while the large diameter annular piston design offers high pre-load, usually approaching or exceeding "ideal". The low pre-load connectors are typically used on conventional subsea completions while high pre-load connectors are used on non-conventional subsea completions where fatigue or high external loading is anticipated such as encountered with director overhead floating production systems or "horizontal" tree applications. These hydraulic systems typically include a reverse acting hydraulic "unlatch or unlock" function piston also. This function effects unlatch of the connector from the wellhead housing so that separation can take place. The unlatch hydraulic piston function typically can generate an unlatching force equal to or greater than the primary latching force. This is necessary to relieve the stored energy in the pre-loaded connection. In addition, a secondary mechanical back-up unlatch system is often required. This is a redundant safety system used should failure of the primary hydraulic unlatch function occur. These mechanical backup systems typically consist of vertically oriented rods which are attached to a vertical motion actuation (lock/unlock) ring. Typically, pulling up on the ring by means of the pull rods will cause the connector to unlatch independent of the hydraulic actuation system. Pulling on the rods is typically accomplished by means of a remotely installed hydraulic tool system or direct pull from an overhead vessel by means of diver or ROV (remotely operated vehicle)-installed pull cables from surface mounted winches.
In the 1980's, it was recognized that the expense of tree connectors with integral hydraulic actuation systems could be reduced in volume applications by removing the hydraulic (latch/unlatch) actuation system from the tree connector and mounting directly to the tree running tool. Latch and unlatch of the tree connector could then be effected by coupling the tree running tool-mounted hydraulic actuation system to the tree connector actuation ring by means of vertically mounted push-pull rods. The tree connector hydraulic actuation system included a reversible means to grab or grip the push-pull rods for purposes of pulling on the rods to effect unlatch. Push on the rods would effect latching. Additional advantages were realized in that failure modes associated with permanently installed hydraulic systems and the need for redundant hydraulic or mechanical unlatching systems were eliminated. This type of design is typically referred to as a "mechanical tree connector". These designs were limited to low pre-load conventional tree applications and design-wise, did not lend themselves to a high-preload application due to general tree running tool geometry restrictions and push-pull rod buckling limitations.
The development of the "through-bore" tree in the mid 1980's, and of the "horizontal" or "side valve" tree in the late 1980's and early 1990's led to increased industry consideration of landing a full-size BOP (Blow-out Preventer) stack on top of a subsea Christmas tree for purposes of workover. The external loading imposed by full-size (typically 18.75 inch) BOP stack and accompanying drilling riser system, especially in deep water, mandated a need for a high pre-load connector in this application. The typical integral hydraulic annular high pre-load connector has been typically employed. However, it was noted by the inventor that it would be desirable to be able to employ a "high preload mechanical connector" in this application also in order to drive the same cost and reliability benefits as was achieved on the low preload conventional applications.